1. Field of the Invention
The present invention relates to a piezoelectric element driving apparatus for driving a plurality of piezoelectric elements that use the piezoelectric effect, in particular, to a piezoelectric element driving apparatus applicable to small printer heads for use with an ink jet printer or the like.
2. Description of the Related Art
In recent years, ink jet printers have been commercially available. Each ink jet printer has ink nozzles from which ink droplets are sprayed to a sheet of paper so as to print characters and images thereon. The ink jet printer uses heating elements and piezoelectric elements that produce the ink droplets and spray them on a sheet of paper. As the piezoelectric elements vibrate, the ink droplets are sprayed. Conventionally, to prevent the printer nozzle from being clogged with ink, piezoelectric nozzles are multi-layered and the spraying of ink droplets is controlled.
The heads of the piezoelectric element driving type ink jet printers use electro-strictness of which mechanical distortion takes place with a crystal such as Rochelle salt or barium titanium in an electric field using piezoelectric effect wherein the dielectric value of a crystal varies as a function of an electric charge on the surface thereof corresponding to an applied mechanical distortion. Using the characteristic that a piezoelectric element is deformed with a voltage, ink droplets are sprayed from nozzles of heads. Since the slope of the voltage and the potential are proportional to the acceleration and the intensity of the deformation of the piezoelectric element, by controlling them, the velocity and diameter of the ink droplets can be varied. Thus, to accurately control the acceleration and size of sprayed ink droplets, it is necessary to properly apply a voltage to the piezoelectric element. to the acceleration and the intensity of the deformation of the piezoelectric element, by controlling them, the velocity and diameter of the ink droplets can be varied. Thus, to accurately control the acceleration and size of sprayed ink droplets, it is necessary to properly apply a voltage to the piezoelectric element.
FIG. 1 shows the structure of a piezoelectric element. Referring to FIG. 1, the piezoelectric element 10 is structured in a rectangular shape. The piezoelectric element 10 has piezoelectric lamination portions 13 and electrodes 11 that are alternately formed. By applying an electric field between the electrodes 11, a vertical mechanical distortion takes place. By applying the mechanical distortion to an ink reservoir of a side 12 disposed adjacent to the electrodes 11, ink droplets are sprayed from the nozzle of the ink reservoir.
When full colors are printed, a plurality of nozzles corresponding to a plurality of ink reservoirs for cyan ink, magenta ink, yellow ink, and black ink are used.
FIG. 2 is a schematic diagram showing the structure of a printer apparatus including a printer head peripheral portion using piezoelectric elements 10. The printer apparatus comprises ink reservoirs 23, a carrier 22, a SP (spacing) motor 26, a shaft 24, an LF (line field) motor 25, a platen 28, and a flat flexible cable (FFC) 27. The carrier 22 moves heads (not shown) in the main scanning direction. The SP motor 26 drives the carrier 22. The shaft 24 is used to move the carrier 22. The LF motor 25 feeds paper 21 in the sub-scanning direction. The FFC 27 bends as the carrier 22 travels.
In the structure shown in FIG. 2, the paper 21 is fed in the sub-scanning direction by the LF motor 25, the platen 28, a feed roller (not shown), and so forth. The carrier 22 is moved along the shaft 24 by the SP motor 26. A drive signal and a control signal are supplied to the heads through the FFC 27 so that ink droplets are sprayed to the paper 21 at a predetermined timing.
In the carrier 22, the ink reservoirs 23 and the heads are connected with respective tubes (not shown). Inks in the ink reservoirs 23 are supplied to the heads. When the piezoelectric elements 10 are driven, they are deformed. Thus, the heads are partly stressed and thereby ink in the heads are partly sprayed from the nozzles. Consequently, an image is formed on the paper 21.
In a conventional piezoelectric element driving circuit, when a drive waveform signal amplified by a power amplifier is sent to a piezoelectric element 10, an RC filter is formed by a total of the resistance of an FFC as a transmission path and the static capacitance of the piezoelectric element. Thus, since a high frequency component of the drive waveform signal is lost, the drive waveform signal cannot be transmitted to the piezoelectric element 10 that requires it.
In particular, as the number of piezoelectric elements becomes large, the capacitance component C of the time constant RC of which the resistance component R and the capacitance component C are multiplied becomes large. Thus, since the time constant xc3x4=RC becomes large, only lower frequency components are transmitted to the piezoelectric elements. Consequently, the piezoelectric effect of the piezoelectric elements that should be driven at high speed is deteriorated. For example, when the piezoelectric elements are used for an ink jet printer, the velocity and size of ink droplets sprayed from the heads cannot be accurately controlled. Thus, the print quality of a print image is deteriorated.
Next, with reference to FIG. 3, a piezoelectric element driving circuit for use with a conventional printer apparatus will be described. The piezoelectric element driving circuit shown in FIG. 3 comprises a drive waveform signal generating circuit 1, a power amplifier 2, a flexible flat cable (FFC) 3, a plurality of head units 4, a plurality of switch devices 5, and a plurality of piezoelectric elements 6. The drive waveform signal generating circuit 1 generates a drive waveform signal for driving a plurality of piezoelectric elements 6. The power amplifier 2 amplifies the drive waveform signal. The FFC 3 connects the power amplifier 2 and the head units 4. The switch devices 5 are disposed in the head units 4. The piezoelectric elements 6 are connected to the switch device 5 of each of the head units 4. The head units 4 are color head units for cyan c, magenta m, yellow y, and black b. Each of the head units 4 has, for example, 32 nozzles. Each piezoelectric element 6 can be represented as a capacitance on an equivalent circuit diagram. Thus, corresponding to 32 nozzles of each color head unit, there is a capacitance of 32 capacitors. By turning on/off switches of each switch device 5 through a controlling circuit (not shown), required piezoelectric elements are driven. In this example, it is assumed that the capacitance of one piezoelectric element 6 is 1 nF.
Next, with reference to FIGS. 4A, 4B, and 4C, the relation of an input waveform signal and an output waveform signal of a conventional piezoelectric element driving circuit will be described.
FIG. 4A shows an output waveform signal of a piezoelectric element driving power amplifier. The output waveform signal of the piezoelectric element driving power amplifier becomes an input waveform signal of an RC filter composed of a resistance component R of an FFC and the capacitance component C of piezoelectric elements.
FIG. 4B shows an output waveform signal in the case that the waveform signal shown in FIG. 4A is input to a load of R=1 ohm and C=10 nF. As is clear from FIG. 4B, the capacitance C of the piezoelectric elements as the load is small, the time content xc3x4=RC=10 nsec, and the output waveform signal is almost the same as the input waveform signal.
FIG. 4C shows an output waveform signal in the case that the waveform signal shown in FIG. 4A is input to a load of which R=1 ohm and C=10xc3x9732xc3x974 colors=1280 nF.
As described above, since the value of the time constant xc3x4=RC is large (namely. xc3x4=RC=1.28 isec), the output waveform signal is largely different from the input waveform signal.
As a related art reference for solving such a problem, Hiroyuki Masunaga has disclosed a piezoelectric element driving circuit as Japanese Patent Laid-Open Publication No. 4-290585. According to the related art reference, a resistor module having a plurality of resistors connected in parallel is disposed. An on/off control signal for driving piezoelectric vibrators is input to the resistor module. An analog switch circuit selects one of resistors from the resistor module corresponding to a selection signal. A signal that passes through the selected resistor is compared with a reference voltage by an operational amplifier circuit. A voltage proportional to the difference is applied to the piezoelectric vibrators. Thus, the deviation of the characteristics of the individual piezoelectric vibrators is adjusted.
However, according to the related art reference, countermeasures against the deterioration of the time constant due to the equivalent capacitance of a plurality of piezoelectric elements have not been disclosed at all. If the piezoelectric elements have individual driving circuits, the time constant does not become large. However, in this case, the size and cost of the circuit become large. In particular, it is necessary for the print heads of the ink jet printer to successively apply impulses to a plurality of laminated piezoelectric elements and simultaneously spray a plurality of streams of ink droplets to a sheet of paper. Thus, when the time constant of the piezoelectric element driving circuit becomes large, the steams of ink droplets that are sprayed delay. Consequently, the print quality of the printer deteriorates and the load of the driving circuit becomes large.
An object of the present invention is to provide a driving circuit for directly driving an applied pulse waveform signal without an increase of the time constant of a plurality of piezoelectric elements that are driven.
A first aspect of the present invention is a piezoelectric element driving circuit for driving a plurality of piezoelectric elements disposed in a plurality of head units, comprising a plurality of power amplifiers for driving the plurality of head units, a plurality of flexible flat cables for connecting the plurality of head units and the plurality of power amplifiers, and a drive waveform signal generating circuit for supplying a drive waveform signal to the plurality of head units through said plurality of power amplifiers, wherein each of the plurality of head units has a switch device for supplying a piezoelectric element current to the plurality of piezoelectric elements, wherein the plurality of power amplifiers are disposed corresponding to the plurality of head units, the plurality of power amplifiers supplying a drive waveform signal that is input from the drive waveform signal generating circuit to the plurality of power amplifiers so as to drive the plurality of head units.
A second aspect of the present invention is a piezoelectric element driving method for driving a plurality of piezoelectric elements disposed in a plurality of head units, each of which has a plurality of power amplifiers for driving the plurality of head units, a plurality of flexible flat cables for connecting the plurality of head units and the plurality of power amplifiers, and a drive waveform signal generating circuit for supplying a drive waveform signal to the plurality of head units, the method comprising the steps of driving the plurality of power amplifiers so as to amplify the drive waveform signal, and causing the plurality of head units to spray large ink droplets, middle ink droplets, or small ink droplets corresponding to the drive waveform signal that is output from the drive waveform signal generating circuit, wherein when the small ink droplets are sprayed, the time constant of the plurality of power amplifiers that are driven allows the number of piezoelectric elements that are simultaneously driven becomes the maximum.
A third aspect of the present invention is a piezoelectric element driving system, used in a printer apparatus, for driving a plurality of piezoelectric elements disposed in a plurality of head units, comprising a plurality of power amplifiers driven for the respective head units, a plurality of flexible flat cables for connecting the plurality of head units and the plurality of power amplifiers, a drive waveform signal generating circuit for supplying a drive waveform signal to the plurality of power amplifiers, print paper to which ink is sprayed from the plurality of head units driven by the plurality of power amplifiers so as to print characters and so forth on the print paper, a mechanical portion for driving the print paper in a sub-scanning direction and traveling the head units in a main scanning direction, wherein the head units spray large ink droplets, middle ink droplets, and small ink droplets driven by the plurality of power amplifiers that amplify the drive waveform signal.
A fourth aspect of the present invention is a piezoelectric element driving circuit for driving a plurality of piezoelectric elements disposed in a plurality of head units, comprising a plurality of power amplifiers for driving the plurality of piezoelectric elements disposed in the plurality of head units, a plurality of first switch devices, disposed corresponding to the plurality of power amplifiers, having a plurality of connection/disconnection switches whose input side is short-circuited, a plurality of flexible cables connected to the connection/disconnection switches of the plurality of first switch devices, and a plurality of second switch devices, disposed corresponding to the plurality of head units, having a plurality of connection/disconnection switches whose input side is connected to the plurality of flexible cables and whose output side is short-circuited and connected to the plurality of head units, wherein the output side of the connection/disconnection switches of the plurality of first switch devices and the input side of the connection/disconnection switches of the plurality of second switch devices are paired and connected, wherein the connection/disconnection of the connection/disconnection switches of the plurality of first switch devices and the plurality of second switch devices is controlled corresponding to the number of piezoelectric elements to be driven so as to decrease the time constant of the plurality of power amplifiers to a predetermined value or less.
In the above-mentioned circuits, driving method, and so forth, the waveform signal generated by the waveform signal generating circuit is amplified by the plurality of power amplifiers. The amplifiers are connected to respective head units. Thus, the load driven by each power amplifier is suppressed. Thus, the distortion of the drive waveform signal against the variation of the load is suppressed.